The present invention relates to a solder ball assembly for use in forming solder bumps on electrodes of a substrate for minute electrical components such as semiconductor packages and electrical connectors as well as on electrodes of a semiconductor element or other minute electrical element.
Many small electronic components, including semiconductor packages such as BGA (ball grid array) packages and CSP's (chip size packages), comprise a substrate having a semiconductor element mounted on one of its sides and having a number of electrodes formed with a prescribed arrangement on its other side or backside.
When such a semiconductor package is mounted on a printed circuit board, the electrodes on the backside of the substrate are soldered to the corresponding electrodes on the printed circuit board for electrical and mechanical connection. This soldering can be performed, for example, using solder bumps which are previously formed on each electrode of the substrate. In this case, after the electrodes of the printed circuit board have been coated with solder paste (or flux) by printing, the semiconductor package is positioned on the printed circuit board in such a manner that the electrodes of the semiconductor package are in alignment with the corresponding electrodes of the printed circuit board, thereby causing each solder bump of the package to contact the solder paste applied to the printed circuit board and holding the package on the board by the adhesion of the solder paste. The printed circuit board, which typically has a plurality of semiconductor packages or other electronic components mounted thereon in this manner, is then heated in a reflow furnace to melt the solder bumps and solder paste to perform soldering. This soldering method for mounting semiconductor packages or other electronic components on a printed circuit board will be referred to as solder bump mounting.
On the inside of such a semiconductor package, a semiconductor element is electrically connected to a substrate. The electrical connection is typically performed either by wire bonding or face down bonding.
In the wire bonding method, a semiconductor element having electrodes is secured to a substrate having corresponding electrodes with an adhesive or by soldering, and then the electrodes of the semiconductor element are electrically connected to the electrodes of the substrate through fine gold wires. Wire bonding can be performed when the semiconductor element has electrodes only along its periphery, but it can not be employed if the semiconductor element has electrodes arranged on the entire surface of one side of the element. In the latter case, if wire bonding is employed, it is difficult to prevent gold wires which are connected to electrodes in a central area of the semiconductor element from contacting gold wires which are connected to electrodes in a peripheral area of the semiconductor element, and the contact between gold wires causes short-circuiting. Another disadvantage of wire bonding is the use of fine gold wires which may be as small as several tens of micrometers in diameter. Such gold wires are extremely expensive due not only to the material cost of gold but also due to the processing costs required to form such fine wires.
In the face down bonding method, including flip chip bonding and some kinds of TAB (tape automated bonding), electrical connection is performed using solder bumps in the same manner as described above for mounting a semiconductor package on a printed circuit board. Thus, after solder bumps have previously been formed on each electrode of a semiconductor element having electrodes on one surface thereof, the semiconductor element is positioned on a substrate (which may be a printed circuit board in the case of flip chip bonding) having electrodes with the same arrangement as the electrodes on the semiconductor element such that the solder bumps of the semiconductor element are in alignment with the electrodes of the substrate. The semiconductor element and the substrate are then heated under pressure to melt the solder bumps, thereby achieving electrical connection between the semiconductor element and its substrate.
Unlike the wire bonding method, even if a semiconductor element has electrodes arranged on the entire surface of one side thereof, there is no concern of short-circuiting taking place due to contact between electrical connections connected to different electrodes on the semiconductor element. The face down bonding method is also advantageous in that the material and production costs of solder bumps are less than those of gold wires and in that all the electrodes on the semiconductor element and the substrate can be electrically connected at the same time, whereby mechanical bonding between the semiconductor element and the substrate is simultaneously achieved. As a result, face down bonding has superior productivity. Furthermore, the length of electrical connections between the semiconductor element and the substrate becomes very short and thereby minimizes a delay in transmission of electrical signals through the connections. In addition, it is possible to increase the density of electrodes or minimize the size of a semiconductor package or other electronic component.
Solder bumps useful for both the above-described solder bump mounting and face down bonding may be formed using solder balls and a mask as disclosed in JP 08-309523 A1 (1996) and JP 2001-196730 A1.
In the method disclosed in JP 08-309523 A1, through-holes having a diameter smaller than that of metal balls which are used to form metal bumps are formed in a mask with the same arrangement as solder bumps which are to be formed. An adherent sheet is attached atop the mask so as to cover the through-holes. Metal balls are then inserted into the through-holes and are held by the adherent sheet in the through-holes. The mask is then positioned on a substrate (or semiconductor element) having electrodes which have been coated with soldering flux in such a manner that the metal balls in the mask are in alignment with the electrodes of the substrate and are held in position by the adhesion of the flux coated thereon. Subsequently, the adherent sheet is peeled from the mask, and the mask is then removed, leaving the metal balls on the electrodes of the substrate. When the metal balls are made of solder, the substrate may be heated in a reflow furnace, for example, to melt the metal (solder) balls and transform them into solder bumps on the electrodes of the substrate.
In this method, after the adherent sheet and the mask are removed, the metal balls are held on the electrodes of the substrate only by the adhesion of the flux applied to the electrode. However, the adhesion of flux is relatively weak. As a result, there is the possibility of the metal balls becoming detached from the electrodes due to shaking or tilting of the substrate during transport or moving away from the electrodes as the flux flows during heating.
JP 2001-196730 A1 discloses a method comprising placing solder balls into holes formed in a mask on a tray. The diameter of the holes is slightly larger than the diameter of the solder balls, but the depth of the holes (which corresponds to the thickness of the mask) is smaller than the diameter of the solder balls. A thermally releasable sheet is then placed over the tray so as to adhere to the solder balls and hold them by the adhesion of the sheet. The sheet holding the solder balls is then pulled up to withdraw the solder balls from the mask and tray and is positioned on a substrate in such a manner that each solder ball contacts a corresponding electrode. The substrate is heated along with the solder balls and the sheet to melt the solder balls and transform them into solder bumps on the electrodes of the substrate. Finally, the thermally releasable sheet, which has lost its adhesion during heating, is removed.
According to this method, since the solder balls are heated while they are held by the thermally releasable sheet, they are to a certain degree prevented from dropping or moving during heating. However, the solder balls are held only by the sheet, which contacts the very small top areas of the balls (i.e., the balls are hanging from the sheet) before the sheet and the solder balls are positioned on the substrate. Therefore, the holding force is not sufficiently strong to prevent the solder balls from dropping from the sheet when the sheet undergoes a mechanical shock or shaking. Thus, it is necessary to perform positioning of the sheet immediately after the solder balls are withdrawn from the tray and mask, and it is practically impossible to store the thermally releasable sheet having solder balls attached thereto for ready use in bump formation at a later time.